1. Field of the Invention
This invention relates to novel low temperature heat transfer methods in a variety of applications.
2. Related Art
Pure or relatively pure helium has excellent heat transfer properties. For example, helium is typically employed to enhance fiber cooling during the optical fiber drawing process because it is chemically inert and because of its heat transfer properties. Of the common pure gases, only hydrogen has a higher thermal conductivity than pure helium. However, hydrogen is not as inert as helium and it is more hazardous to employ in certain heat transfer applications than any inert gas. So, hydrogen is typically avoided as a gaseous heat transfer medium in some (but not all) cooling or heating process applications. Substantially pure argon, nitrogen, or carbon dioxide are also typically avoided as gaseous cooling agents because their heat transfer properties are not as good as either hydrogen or helium, but they are safer to use than hydrogen and much cheaper than helium.
Typical impurities in the helium used in heat transfer processes are due to minor impurities initially present within the source of xe2x80x9cpurexe2x80x9d helium as well as contamination by infiltration of other species into the helium that is used to transfer heat between the helium and the item or material being cooled or heated. These impurities often consist primarily of nitrogen and oxygen with much smaller concentrations of argon, carbon dioxide, and water vapor as well as even smaller concentrations of other gaseous constituents normally found in air. These impurities are generally tolerated because they are difficult and/or costly to avoid.
It is generally accepted that binary mixtures of helium (or hydrogen) with other gases will have better heat transfer coefficients than the pure gases themselves. See, for example, M. R. Vanco, xe2x80x9cAnalytical Comparison of Relative Heat-Transfer Coefficients and Pressure Drops of Inert Gases and Their Binary Mixtures, NASA TN D2677 (1965); F. W. Giacobbe, xe2x80x9cHeat Transfer Capability of Selected Binary Gaseous Mixtures Relative to Helium and Hydrogenxe2x80x9d, Applied Thermal Engineering Vol. 18, Nos. 3-4, pp.199-206 (1998); R. Holoboffet al., xe2x80x9cGas Quenching With Heliumxe2x80x9d, Advanced Materials and Processes, Vol. 143, No. 2, pp.23-26 (1993). In particular, Holoboff et al. noted that in the context of a heat treating furnace, by changing to an optimum helium/argon mixture, a customer was able to heat treat parts that could not be processed as rapidly when using argon alone, while maintaining costs at a fraction of that for using 100% helium. In a separate example the same authors also recognized the benefits of increasing the fan speed (gas circulation velocity) on cooling rate for pure helium and for pure nitrogen. However, there is no teaching or suggestion of the influence of heat transfer fluid mixture velocity on cooling rate for optimized mixtures of heat transfer fluid.
For illustrative purposes, and according to earlier theories, the relative heat transfer capability of helium plus one other noble gas compared to pure helium may be seen in FIG. 1. In FIG. 1, pure helium has been arbitrarily assigned a relative heat transfer capability of 1.0 in order to deliberately avoid the use of a more complicated system of SI heat transfer units. Therefore, if a binary gas mixture containing helium has a heat transfer capability of 2.0 (relative to pure helium), it is assumed from this data that the gas mixture will be 2.0 times more effective in any heat transfer process employing that gaseous mixture instead of pure helium alone. And, as a simplified illustration of the potential helium savings using this data, if the best binary gas mixture contained only 50 percent (by volume or mole fraction) helium plus 50 percent of some other gas, only xc2xd of that gas mixture would be needed to perform the same cooling function as the pure helium alone. Therefore, only 25 percent of the helium that would have been required for a particular heat exchange process using pure helium would be needed during the same cooling process employing the gas mixture.
In FIG. 2, and also according to earlier theories, the optimum composition and approximate relative heat transfer capability of hydrogen plus one noble gas with respect to pure helium is illustrated. In FIG. 2, pure helium has also been arbitrarily assigned a relative heat transfer capability of 1.0. So, if a binary gas mixture containing only hydrogen and argon (but no helium) has a heat transfer capability of 1.4 (relative to pure helium), that gas mixture presumably will be 1.4 times more effective in any heat transfer process employing that gaseous mixture instead of pure helium alone. And, since no helium is required to produce this effect, the helium usage is cut to zero. Furthermore, since hydrogen and argon are typically much less expensive than helium, the overall cost of the hydrogen/argon coolant gas stream will tend to be negligible compared to a pure (or relatively pure) helium coolant gas steam.
It should be emphasized that the data presented in FIGS. 1 and 2 are theoretical and based on turbulent flow for all the gases and gas mixtures considered. However, in the seminal work of R. B. Bird, W. E. Stewart, and E. N. Lightfoot, Transport Phenomena, pp. 392-393 (1960) it was pointed out that xe2x80x9cthe heat-transfer coefficient depends in a complicated way on many variables, including the fluid properties (k, xcexc, xcfx81, Cp), the system geometry, the flow velocity, the value of the characteristic temperature difference, and the surface temperature distribution.xe2x80x9d In engineering design, therefore, use of constant property idealization frequently leads to either a greater built in safety factor, or a dangerous situation if the other extreme is taken. See D. M. McEligot, et al., xe2x80x9cInternal Forced Convection to Mixtures of Inert Gasesxe2x80x9d, Int. J. Heat Mass Transfer, Vol. 20, pp. 475-486 (1977).
Everyone agrees that helium is an expensive fluid. While it is inert, it is a non-renewable resource. Once it escapes to the atmosphere it is not recoverable. Helium is commonly recycled, sometimes after purification, such as described in U.S. Pat. Nos. 5,897,682 and 6,092,391. However this requires expensive compression and/or cryogenic equipment. Indeed, as noted by K. Bammert et al., xe2x80x9cThe Influence of Hexe2x80x94Ne, Hexe2x80x94N2, and Hexe2x80x94CO2 Gas Mixtures on Closed-Cycle Gas Turbinesxe2x80x9d, ASME paper 74-GT-124 (1974), while pure helium is often considered the best gaseous fluid in terms of heat transfer efficiency (except for pure hydrogen) and therefore heat exchange units may be particularly compact, the size of compression equipment required to compress the gas is prohibitive in many applications, such as space travel. Thus, the expense of the heat transfer fluid (helium) is combined with a relatively large expense for compression equipment, even though heat transfer equipment may be smaller.
In light of the unexpected nature of heat transfer coefficients of gases and gas mixtures, it would be advantageous in many heat transfer situations common in engineering to employ a substantially pure heat transfer fluid, even though the substantially pure fluid heat transfer coefficient is less than the heat transfer coefficient of a mixture of that fluid with another heat transfer fluid under certain conditions.
In accordance with the present invention, a first aspect of the invention is a method of cooling an object, the method comprising: a) contacting a heat transfer fluid with a liquid cryogen to form a precooled heat transfer fluid; and b) contacting the object with the precooled heat transfer fluid to form a cooled object and a recycle flow of said heat transfer fluid, the precooled heat transfer fluid consisting essentially of a compound selected from the group consisting of substantially pure hydrogen, substantially pure helium, substantially pure argon, substantially pure nitrogen, and substantially pure carbon dioxide, and wherein the contacting the object is selected from the group consisting of directly contacting the object, indirectly contacting the object, and combinations thereof.
Preferred are methods wherein the precooled heat transfer fluid has a concentration of at least about 90 mole percent, more preferably at least about 95 mole percent, more preferably at least about 99 mole percent.
Preferred are methods wherein a temperature of the cooled object, and a temperature of the precooled heat transfer fluid are monitored. Particularly preferred are methods wherein these temperatures are used to control flow of the precooled heat transfer fluid, flow of the cryogen, and the recycle flow of heat transfer fluid.
Other preferred methods are those wherein the object is substantially stationary and the precooled heat transfer fluid moves past the object.
Yet other preferred methods are those wherein the object moves continuously through a space and the precooled heat transfer fluid moves through the space during the step of contacting the object with the precooled heat transfer fluid.
Preferred methods also include methods wherein the precooled heat transfer fluid exchanges heat with a second heat transfer fluid, and the second heat transfer fluid directly contacts the object.
Preferably, the object is a substantially cylindrical object traversing through a substantially confined space, more preferably an optical fiber traversing through a heat exchanger.
More preferably, the methods comprise spraying the precooled heat transfer fluid onto the object.
A second aspect of the invention is a method of cooling a substantially cylindrical object traversing a substantially confined space, the method comprising: a) directly contacting a heat transfer fluid with the substantially cylindrical object inside of the confined space, the confined space defined by a substantially cylindrical tube having a tube inlet and a tube outlet, the substantially cylindrical object entering the tube inlet and exiting the tube outlet, the substantially cylindrical tube having a heat transfer fluid inlet near the tube outlet and a heat transfer fluid outlet near the tube inlet; and b) substantially maintaining a temperature of the heat transfer fluid contacting the substantially cylindrical object at a precooled temperature by indirectly or directly contacting the heat transfer fluid with a cryogen to form a cooled substantially cylindrical object and an exit flow of the heat transfer fluid, the heat transfer fluid consisting essentially of a compound selected from the group consisting of substantially pure hydrogen, substantially pure helium, substantially pure argon, substantially pure nitrogen, and substantially pure carbon dioxide.
Preferred methods within this aspect of the invention are those wherein step b) comprises monitoring the temperature of the heat transfer fluid and a temperature the exit flow of the heat transfer fluid; methods wherein step b) comprises monitoring a flow of the cryogen; methods wherein a temperature of the cooled substantially cylindrical object is monitored; methods wherein a diameter of the cooled substantially cylindrical object is monitored; methods wherein a flow of the cryogen and a flow of the heat transfer fluid are controlled, and combinations of these.
As used herein the terms xe2x80x9cprecooledxe2x80x9d and xe2x80x9cprecooled temperaturexe2x80x9d mean the heat transfer fluid is cooled significantly below ambient temperature (typically 25xc2x0 C.), preferably more than 20xc2x0 C. below ambient, more preferably more than 50xc2x0 C. below ambient, and most preferably at least 100xc2x0 C. below ambient temperature. As used herein the term xe2x80x9csubstantially purexe2x80x9d means that the heat transfer fluid does not contain an amount of another gas or liquid that would significantly change the heat transfer characteristics of the selected gas. As used herein, the term xe2x80x9cheat transfer fluidxe2x80x9d means a gas, or mixture consisting of a gas and a liquid, or a liquid.
Surprisingly, precooled versions of substantially pure gas, such as hydrogen, have as good or better heat transfer characteristics than mixtures of hydrogen and another gas, say helium, at bulk velocities where one of skill in the art would have expected (without knowledge of the present invention) that the mixtures would have better heat transfer characteristics. This can have enormous consequences in situations where one gas of the gas mixture is not readily available, or available but too expensive to obtain or store, or where mixing and mixture monitoring equipment is unavailable, or potentially unreliable. The teachings of the invention allow heat transfer designers another option, use of precooled, substantially pure heat transfer fluids, rather than heat transfer mixtures. While there is a cost in precooling the heat transfer fluids to the precooled temperature, the simplicity of using a single heat transfer fluid, rather than mixing, may be attractive to some heat transfer designers.
Further aspects and advantages of the methods of the invention will become apparent after reading the following description and claims.